1. Field of the Invention
The present invention relates to a method of forming a mask of undercut shape, and a method of forming a patterned thin film and a method of fabricating a micro device in which a patterned thin film is formed through the use of the mask.
2. Description of the Related Art
In micro devices having a thin film formed into a specific pattern (called a patterned thin film in this application), the patterned thin film is formed by using, for example, a patterned resist layer as a mask. A micro device means a small-size device fabricated through the use of thin-film forming techniques. Examples of the micro device include semiconductor devices, thin-film magnetic heads, transducers and actuators incorporating thin films.
As disclosed in Published Unexamined Japanese Patent Application (KOKAI) No. Heisei 9-96909 (1997), for example, methods such as etching, liftoff, and a combination of etching and liftoff (hereinafter called a combination method) are available to form a patterned thin film on a base layer by using a patterned resist layer as a mask.
To perform the aforementioned methods, it is preferable to use a mask of undercut shape, that is, a mask in which the bottom portion has a width smaller than that of the upper portion. One of known methods for forming such a mask is to use a two-layer resist, as disclosed in Published Unexamined Japanese Patent Application (KOKAI) No. Heisei 2-17643 (1990).
The method of forming a mask of undercut shape through the use of a two-layer resist will now be briefly described with reference to FIG. 35 to FIG. 38. In this method, as shown in FIG. 35, a first layer 302 that is the lower layer of the two-layer resist is first formed on a base layer 301. Next, as shown in FIG. 36, a second layer 303 that is the upper layer of the two-layer resist is formed on the first layer 302. The second layer 303 is made of a resist. The resist can be any of a negative type resist, a positive type resist, a novolak type resist, a chemically amplified type resist, etc. The first layer 302 is made of a material that is soluble in a developer. The first layer 302 is made of polymethylglutarimide, for example.
Next, as shown in FIG. 37, the second layer 303 is exposed to light for forming a latent image of specific pattern through a mask 304, thereby forming a latent image of specific pattern on the second layer 303. Light used for the exposure may be any of ultraviolet rays, a beam of an excimer laser, an electron beam, etc. Next, as shown in FIG. 38, the second layer 303 having undergone the exposure is developed and part of the first layer 302 is dissolved in the developer to form a mask made up of an upper layer 306 made of the remainder of the second layer 303 and a lower layer 305 made of the remainder of the first layer 302. The development is continued until a width W1 of the first layer 302 on the lower side becomes smaller than a width W2 of the second layer 303 on the upper side. A mask of undercut shape is thus obtained.
In the above-described conventional method for forming a mask of undercut shape through the use of the two-layer resist, when developing the second layer 303, the first layer 302 is dissolved until the width of the first layer 302 becomes smaller than that of the second layer 303. For this reason, in the conventional method, the portion of the first layer 302 bonded to the base layer 301 is reduced in area during the development, which not only reduces adhesiveness between the first layer 302 and the base layer 301, but also reduces mechanical strength of the first layer 302 and the second layer 303. Further, during the development, the flow of the developer or a cleaning solution applies a force to the first and second layers 302 and 303. Therefore, this conventional method has a problem that the first and second layers 302 and 303 are readily peeled off or damaged during the development. This problem becomes noticeable when an attempt is made to obtain a mask having a smaller width. Hence, a mask having a sufficiently small width cannot be obtained by this conventional method. Therefore, it is difficult to obtain a fine patterned thin film by using the mask formed by the above-described method.